The present invention relates to a semiconductor device in use for an active matrix display device, wherein a liquid crystal is driven by a thin film transistor, particularly, a driver monolithic type liquid crystal display device, wherein a peripheral drive circuit is on the same substrate, and the like and a method of manufacturing the same.
Among thin type liquid crystal display devices with low power consumption, those using a thin film transistor (hereinafter, referred to as TFT) for a drive element are mainly used for a display unit of a personal computer or the like, a portable TV (television) and the like due to its high performance such as high contrast, high response speed and so forth, and its market size has been greatly expanded in recent years.
The TFTs in use for a liquid crystal display device include the one wherein a CG silicon film is used as a semiconductor of its active portion. As described in Japanese Patent Laid-Open Publication No. 6-244103, this CG silicon film is a silicon film having excellent crystallinity obtained by depositing a trace of a kind of metal element such as nickel Ni or the like on the surface of an amorphous silicon film and thereafter heating the film.
Since the CG silicon film has lower power consumption and faster response than an amorphous silicon film and a polycrystalline silicon and has an advantage that a future sheet computer can be manufactured by utilizing its high mobility, it is considered promising as a material for manufacturing a next-generation driver monolithic type liquid crystal display device.
The CG silicon film is a crystalline silicon film formed by adding a metal element for promoting crystallinity into an amorphous silicon film and thereafter heating the film. A method of removing the metal element introduced into silicon Si at this time is disclosed in Japanese Patent Laid-Open Publication No. 10-223533. In this Japanese Patent Laid-Open Publication No. 10-223533, a part of the formed CG silicon film is doped with a 5th group element P (phosphorous) in high concentration and subsequently heat treated, thereafter the metal element is removed from a region to be used as an active portion of the TFT by gettering the metal from the region doped with P (phosphorous).
Furthermore, methods of forming a CG silicon film include a method called longitudinal growth and a method called lateral growth. The longitudinal growth is a method wherein a metal element is directly added to the whole surface of an amorphous silicon film and then heated to allow crystal growth. Meanwhile, the lateral growth is a method wherein, for example, a SiO2 film formed on an amorphous silicon film is photopatterned so that a part of the amorphous silicon film is exposed, a metal element is added to the exposed portion and the film is heated so that crystals are grown in a direction horizontal with respect to a substrate into a portion where the amorphous silicon film is not exposed.
Here, the longitudinal growth, which is the most relevant to the present invention, will be explained in detail.
FIGS. 11A-11D show how the longitudinal growth occurs in a CG silicon film. FIGS. 11A-11D are all plan views viewed from the film surface side. In FIGS. 11A-11D, reference numeral 71 denotes an amorphous silicon film. Reference numeral 72 denotes a Si crystal, which is to be a nucleus. Reference numeral 73 denotes a CG silicon crystal (also referred to as xe2x80x9cdomainxe2x80x9d). Reference numeral 74 denotes a domain boundary.
First, as shown in FIG. 11A, a catalyst metal element is added to the surface of an amorphous silicon film 71 on a quartz substrate.
Subsequently, when solid-phase crystal growth is allowed in this amorphous silicon film 71 at temperature of about 600xc2x0 C. for about 1 hour, a Si crystal 72 which is to be a nucleus is formed at several points on the quartz substrate as shown in FIG. 11B. The generation density of the Si crystals 72 which are to be nuclei is affected by the quality of the amorphous silicon film 71, the concentration of an added metal element and so forth.
When solid-phase crystal growth is further allowed for a long time, CG silicon crystals 73 grow radially from the crystal to be a nucleus as a center as shown in FIG. 11C. The region of these CG silicon crystals grown from one nucleus as a center is referred to as a domain. While the inside of this domain 73 is polycrystalline, this can be considered as a so-to-speak quasi-single crystal since these crystals are known to have better continuity than those of p-Si.
When solid-phase crystal growth is further allowed for a long time, the grown domains 73 are finally bumped against each other as shown in FIG. 11D. Then, the whole surface of the substrate becomes a CG silicon film, and the growth finishes. In FIG. 11D, a place where the domains 73 are bumped against each other is referred to as a domain boundary 74. The size of these domains depends on the formation conditions, but a big domain may exceed 200 xcexcm in diameter.
Meanwhile, when a TFT is formed by the CG silicon film, either the longitudinal growth or the lateral growth is employed. However, when a TFT is formed by lateral growth, the amount of a catalyst metal to be introduced must be about ten times more than the amount introduced for longitudinal growth. The reason for this is that, if the amount of the catalyst metal to be introduced is reduced, a distance of crystal growth in the horizontal direction becomes shorter, or a portion where few crystals grow is generated. Therefore, the CG silicon film by lateral growth inevitably contains a more amount of the catalyst metal than the silicon film by longitudinal growth. Thus, it is highly likely that the amount of the residual catalyst metal after gettering is naturally more in the laterally grown CG silicon film than in the longitudinally grown CG silicon film. Since metal element residues in such a silicon film adversely affect TFT characteristics (particularly OFF characteristics), changes with time such as deterioration or the like and so forth, a more excellent TFT can be obtained by reducing the metal element in the TFT to a minimum.
Consequently, the longitudinally grown CG silicon film is considered to be more suitable to formation of an active region of a TFT than the laterally grown CG silicon film.
Meanwhile, when a TFT is formed by the CG silicon film, either the longitudinal growth or the lateral growth is employed. However, when a TFT is formed by lateral growth, the amount of a catalyst metal to be introduced must be about ten times more than the amount introduced for longitudinal growth. The reason for this is that, if the amount of the catalyst metal to be introduced is reduced, a distance of crystal growth in the horizontal direction becomes shorter, or a portion where few crystals grow is generated. Therefore, the CG silicon film by lateral growth inevitably contains a greater amount of the catalyst metal than the silicon film by longitudinal growth. Thus, it is highly likely that the amount of the residual catalyst metal after gettering is naturally more in the laterally grown CG silicon film than in the longitudinally grown CG silicon film. Since metal element residues in such a silicon film adversely affect TFT characteristics (particularly OFF characteristics), changes with time such as deterioration or the like and so forth, a more efficient TFT can be obtained by reducing the metal element in the TFT to a minimum.
Since each of the TFTs for pixels of the display unit is responsible for the display of a different pixel, the differences in characteristics lead to differences in an electric potential applied to each pixel electrode, a charge holding time or the like, which are directly reflected on differences in transmittance of liquid crystal. That is, in display on a TFT panel using a longitudinally grown CG silicon film, light transmittance of each pixel varies depending on the presence or absence of a domain boundary in a TFT active region of the display unit. Therefore, there is high possibility that the variation results in uneven display.
Furthermore, a leak current of the TFT for a pixel of a display unit in an OFF state must be kept lower than that of the TFT for a peripheral drive circuit. However, when the TFT for a pixel is formed by introducing a catalyst metal, there is a possibility that a residual metal exists even after a step of removing the catalyst metal by gettering as described above, and the residual metal may deteriorate the OFF characteristics of the TFT.
To avoid the uneven display of a liquid crystal display region and the deterioration of TFT OFF characteristics, a method of forming a TFT in a state of an amorphous silicon film without introducing a catalyst metal into a screen is suggested as described in Japanese Patent Laid-Open Publication No. 8-78689. However, when an amorphous silicon film forms a TFT active region in the screen, TFT characteristics are naturally deteriorated compared with a TFT formed by polycrystalline silicon or CG silicon.
Currently, a display unit of a liquid crystal panel is being developed towards high intensity and high precision. Since sufficient ON characteristics cannot be obtained from a TFT using an amorphous silicon film, formation of a TFT for display unit by using an amorphous silicon film is difficult in practice.
Furthermore, Japanese Patent Laid-Open Publication No. 9-45931 discloses a method of forming TFTs in a screen by crystalline silicon, which is not CG silicon, and using CG silicon for a peripheral drive circuit. However, in this method, a residual concentration of the catalyst metal element in the peripheral drive circuit is high. Since the catalyst metal element is considered to affect TFT characteristics and reliability of the peripheral drive circuit, practical use is still difficult.
Accordingly, an object of the present invention is to provide a semiconductor device and a method of manufacturing the same by which a driver monolithic type liquid crystal display device with high intensity, high precision and uniform characteristics can be achieved.
In order to achieve the above object, there is provided a semiconductor device including a display unit having pixel electrodes arranged in a matrix and transistors for a pixel connected to the pixel electrodes and a peripheral drive circuit having a transistor for a peripheral drive circuit provided outside the display unit, wherein
the transistor for a peripheral drive circuit has a first crystalline silicon film, which is to be a crystal grown active region formed by introducing a catalyst metal into an amorphous silicon film and heating the film;
the transistor for a pixel has a second crystalline silicon film, which is to be an active region, formed by crystallizing the amorphous silicon film without introducing the catalyst metal; and
the concentration of the catalyst metal in the second crystalline silicon film is lower than the concentration of the catalyst metal in the first crystalline silicon film.
In one embodiment of the present invention, the concentration of the catalyst metal in the first crystalline silicon film is in the range of 1xc3x971013 atoms/cm3 or higher and lower than 1xc3x971015 atoms/cm3.
According to a semiconductor device of the above constitution, in the above transistor for a peripheral drive circuit, in which a first crystalline silicon film (CG silicon film) wherein crystals are grown by introducing a catalyst metal into the amorphous silicon film and thereafter heating the film is used as an active region, the concentration of the catalyst metal in the first crystalline silicon film is in the range of 1xc3x971013 atoms/cm3 or higher and lower than 1xc3x971015 atoms/cm3. Therefore, the catalyst metal does not greatly affect transistor characteristics, and hence a high ON-current operation can be performed. It is noted that, in the transistor for a pixel, wherein a second crystalline silicon film, in which an amorphous silicon film is crystallized without introducing a catalyst metal, is used as an active region, a low OFF-current operation can be performed since the concentration of the catalyst metal in the second crystalline silicon film is lower than the concentration of the catalyst metal in the first crystalline silicon film. Thus, a driver monolithic type liquid crystal display device with high intensity, high precision and uniform characteristics can be achieved.
The concentration of the catalyst metal in the first crystalline silicon film in the range of 1xc3x971013 atoms/cm3 or higher and lower than 1xc3x971015 atoms/cm3 is measured by a measuring method called ICP (Inductively Coupled Plasma)xe2x80x94MS (Mass Spectrometer).
The applicants of the present invention used nickel Ni as a catalyst metal, formed the whole surface of a substrate by a CG silicon film and removed the CG silicon film by gettering. Then, they measured the amount of nickel Ni element in this sample CG silicon film by atomic absorption photometry. As a result, the amount of the nickel Ni element in the CG silicon film was 4xc3x97109 atoms/cm2 or less in average over the whole substrate surface. Since the CG silicon film had a thickness of 70 nm and a area ratio of 65% on the substrate at this time, the concentration of the catalyst metal in the CG silicon film was converted to 8.79xc3x971014 atoms/cm3 or lower, which satisfied the above catalyst metal concentration in the range of 1xc3x971013 atoms/cm3 or higher and lower than 1xc3x971015 atoms/cm3.
In one embodiment of the present invention, the transistor for a pixel has an active region, a pair of high-concentration impurity regions, which are to be source regions, and drain regions formed on both sides of the active region and a pair of low-concentration impurity region formed between the source region and the active region and between the drain region and the active region.
According to the semiconductor device of the above embodiment, a structure having a pair of low-concentration impurity regions formed between the source region and the active region and between the drain region and the active region is called an LDD (lightly doped drain) structure. Since an OFF current can be reduced without reducing the ON current of the transistor, this structure is very effective as a transistor structure for a pixel.
In one embodiment of the present invention, the transistor for a pixel has two active regions formed with a prescribed gap therebetween, a pair of high-concentration impurity regions to be used as a source region and a drain region formed on both sides of the two active regions, a pair of low-concentration impurity regions formed between the high-concentration impurity region to be used as the source region and one of the active regions adjacent to the high-concentration impurity region and between the high-concentration impurity region to be used as the drain region and the other active region adjacent to the high-concentration impurity region, and an impurity region formed in a region sandwiched between the two active regions.
According to the semiconductor device of the above embodiment, a structure having the two active regions, a pair of high-concentration impurity regions, a pair of low-concentration impurity regions and an impurity region between the two active regions is a structure wherein two LDD-structure transistors are arranged in series (hereinafter, referred to as a dual LDD structure). The transistor in this dual LDD structure is characterized by uniformed and stabilized characteristics of a transistor for a pixel. That is, when the formed transistors are compared with each other, there are more or less variations in characteristics. When one transistor is in charge of one pixel, a problem of uneven display in a liquid crystal panel or the like may occur due to the characteristic variation. Therefore, characteristics are balanced by two transistors by employing transistors in this dual LDD structure, and hence uniformity of the whole transistor is improved and occurrence of a problem such as uneven display or the like can be prevented.
In one embodiment of the present invention, the impurity region formed in the region sandwiched between the two active regions contains at least either one of:
an impurity region having the same concentration as that of the pair of high-concentration impurity regions, which are to be the source region and the drain region, or an impurity region having the same concentration as that of the pair of low-concentration impurity regions.
According to the semiconductor device of the above embodiment, for example, the impurity region formed in a region sandwiched between the above two active regions is an impurity region having the same concentration as that of the pair of high-concentration impurity regions to be used as the source region and the drain region. Therefore, two transistors are connected in series so that a dual LDD structure is formed. Furthermore, the impurity region formed in a region sandwiched between the two active regions is an impurity region having the same concentration of that of the pair of low-concentration impurity regions. Therefore, a dual LDD structure is similarly formed. Furthermore, this structure is similarly formed when the impurity region formed in a region sandwiched between the two active regions includes both an impurity region having the same concentration as that of the pair of high-concentration impurity regions and an impurity region having the same concentration as that of the pair of low-concentration impurity regions.
In one embodiment of the present invention, the active region of the transistor for a peripheral drive circuit formed in the first crystalline silicon film and the active region of the transistor for a pixel formed in the second crystalline silicon film have a distance of at least 100 xcexcm or longer therebetween.
This is because, when a catalyst metal is introduced into a region where the first crystalline silicon film is to be formed and thereafter the region is heated, a CG silicon film grown from the region into which the catalyst metal has been introduced is grown a certain distance and enters a region where a second crystalline silicon film is formed and into which the catalyst metal is not introduced originally. That is, a crystalline silicon film containing an originally unintended catalyst metal element is formed between the first crystalline silicon film and the second crystalline silicon film. This region in the second crystalline silicon film, into which the CG silicon is grown into, is less than 100 xcexcm as a result of observation in experiments so far although there are variations. The crystalline silicon film of this region, into which the CG silicon is grown into, has a composition different from those of the first crystalline silicon film and the second crystalline silicon film, and hence characteristics of this silicon film are different from those of the other films. Therefore, the silicon film of this region cannot be used for a transistor for a peripheral drive circuit or a transistor for a display unit.
Therefore, according to the semiconductor device of the above embodiment, by keeping a distance of at least 100 xcexcm or more between an active region of a transistor for a peripheral drive circuit formed by the first crystalline silicon film and an active region of a transistor for a pixel formed by the second crystalline silicon film, the silicon film in the region, into which CG silicon is grown into, can be reliably prevented from being used as a transistor for a peripheral drive circuit or a transistor for a display unit.
Also, there is provided a semiconductor device manufacturing method of manufacturing the above semiconductor device, wherein,
the catalyst metal in the first crystalline silicon film is removed by gettering.
According to the above method of manufacturing a semiconductor device, the concentration of the catalyst metal in the first crystalline silicon film can be made in the range of 1xc3x971013 atoms/cm3 or higher and lower than 1xc3x971015 atoms/cm3 by removing the catalyst metal in the first crystalline silicon film by gettering.
There is provided a method of manufacturing a semiconductor device including a display unit having pixel electrodes arranged in a matrix and transistors for a pixel connected to the pixel electrodes and a peripheral drive circuit having a transistor for a peripheral drive circuit provided outside the display unit, which comprises the steps of:
forming an amorphous silicon film over the whole surface of a substrate;
forming a protection film on a region to be used as a display unit in the amorphous silicon film;
after forming the protection film, introducing a catalyst metal into the region to be used as a peripheral drive circuit in the amorphous silicon film except for the region covered with the protection film;
after introducing a catalyst metal into the region to be used as the peripheral drive circuit, forming a crystalline silicon film by heating the amorphous silicon film to grow crystals in the region to be used as a peripheral drive circuit and the region to be used as a display unit; and
removing the catalyst metal in the crystalline silicon film by gettering.
According to the above method of manufacturing a semiconductor device, a protection film is formed on the region to be used as a display unit in an amorphous silicon film formed over the whole substrate surface, and then a catalyst metal is introduced into the region to be used as a peripheral drive circuit in the amorphous silicon film except for the region covered with the protection film. Subsequently, the amorphous silicon film is heated so that crystal growth is allowed in the region to be used as a peripheral drive circuit to form a crystalline silicon film (longitudinally grown CG silicon film), while crystal growth is allowed in the region to be used as a display unit to form a crystalline silicon film (polycrystalline silicon film). Then, the catalyst metal in the crystalline silicon film is removed by gettering. Thus, by using a longitudinally grown CG silicon film formed by directly adding a catalyst metal as a transistor for a peripheral drive circuit and a polycrystalline silicon film formed without adding a catalyst metal as a transistor for a display unit, a driver monolithic type liquid crystal display device with stable operations and high display quality can be manufactured.
In one embodiment of the present invention, the catalyst metal is at least one kind selected from Fe, Co, Ni, Pd, Pt, Cu, Au, In and Sn.
According to the method of manufacturing a semiconductor device of the above embodiment, as long as at least one or a plurality of kinds of elements selected from Fe, Co, Ni, Pd, Pt, Cu, Au, In and Sn are used as the catalyst elements, an effect of promoting crystallization can be obtained by using only a trace thereof. In particular, among these catalyst elements, a most remarkable effect can be obtained when Ni is used.
In one embodiment of the present invention, the protection film formed on the region to be used as a display unit in the amorphous silicon film is silicon oxide having a film thickness of 100 nm or larger.
According to the method of manufacturing a semiconductor device of the above embodiment, by forming silicon oxide having a film thickness of 100 nm or larger as a protection film formed on the region to be used as a display unit in the amorphous silicon film, the region to be used as a display unit in the amorphous silicon film is masked, thereby reliably preventing the introduction of the catalyst metal into the region to be used as a display unit when the catalyst metal is introduced.
In one embodiment of the present invention, in the step of growing crystals in the region to be used as a peripheral drive circuit and the region to be used as a display unit by heating the amorphous silicon film to form a crystalline silicon film, the heating temperature is in the range of 500-700xc2x0 C.
According to the method of manufacturing a semiconductor device of the above embodiment, favorable silicon crystallization can be achieved by heating the amorphous silicon film at temperature in the range of 500-700xc2x0 C.
In one embodiment of the present invention, the step of removing the catalyst metal from the crystalline silicon film by gettering includes steps of:
introducing a 15th group element selectively into the crystalline silicon film;
moving the catalyst metal in the crystalline silicon film by heating the crystalline silicon film to a region of the crystalline silicon film into which the 15th element is selectively introduced; and
after moving the catalyst metal in the crystalline silicon film to a region in the crystalline silicon film into which the 15th group element is selectively introduced, etching is performed to remove the region in the crystalline silicon film to which the catalyst metal is moved.
According to the method of manufacturing a semiconductor device of the above embodiment, a 15th group element is selectively introduced into the crystalline silicon film, and thereafter, by heating the crystalline silicon film, the catalyst metal in the crystalline silicon film is moved to a region in the crystalline silicon film into which the 15th group element is selectively introduced. Thus, since the region in the crystalline silicon film into which the 15th group element is selectively introduced, that is the region to which the catalyst metal is moved, is removed by etching, little catalyst metal is contained in the remaining crystalline silicon film.
In one embodiment of the present invention, the region in the crystalline silicon film into which the 15th group element is selectively introduced is formed not only in the region to be used as a peripheral drive circuit into which the catalyst metal is introduced, but also in the region to be used as a display unit into which the catalyst metal is not introduced.
Since the catalyst metal added to the first crystalline silicon film is subjected to a heating process and the like, the catalyst metal may be introduced into the second crystalline silicon film by diffusion.
Therefore, according to the method of manufacturing a semiconductor device of the above embodiment, by also providing a portion for the second crystalline silicon film with a gettering region to perform gettering of the catalyst metal, deterioration of transistor characteristics due to the catalyst metal can be prevented and a transistor for a pixel with a low OFF current can be manufactured.
In one embodiment of the present invention, in the step of moving the catalyst metal in the crystalline silicon film by heating the crystalline silicon film to the region in the crystalline silicon film into which the 15th group element is selectively introduced, the heating temperature is in the range of 500-800xc2x0 C.
According to the method of manufacturing a semiconductor device of the above embodiment, by heating the crystalline silicon film at temperature in the range of 500-800xc2x0 C., the catalyst metal can be effectively removed by gettering.
In one embodiment of the present invention, after the step of growing crystals in the region to be used as a peripheral drive circuit and the region to be used as a display unit by heating the amorphous silicon film to form a crystalline silicon film, a step of reducing the film thickness of at least a region to be used as the active region of a display unit in the crystalline silicon film is included.
Characteristics of a transistor for a pixel made of a crystalline silicon film formed by allowing crystal growth in the region to be used as a display unit depend on the initial thickness of the amorphous silicon film before the film thickness is reduced and the final film thickness of the active region. FIG. 12 shows a relationship of ON-current and OFF-current characteristics of the transistor for a pixel to the initial thickness of the amorphous silicon film before the film thickness is reduced. In the transistor used in this measurement, both the active regions are formed in the same film thickness of 40 nm. The gate voltage at ON is set to be 5 V, the gate voltage at OFF is set to be xe2x88x9210 V and the drain voltage is set to be 9 V (at both ON and OFF). It is noted that a current value of, for example, xe2x80x9c1.Exe2x88x9207xe2x80x9d represents 1.0xc3x9710xe2x88x9207 in FIG. 12.
FIG. 12 shows that, when the initial thickness of the amorphous silicon film is larger, both the ON and OFF characteristics of the transistor are more favorable. However, the OFF characteristics of the transistor are more favorable due to a lower current value when the film thickness of the active region is smaller.
According to the method of manufacturing a semiconductor device of the above embodiment, when the initial thickness of the amorphous silicon film is made larger, the film thickness needs to be reduced to make the film thickness of the active region thinner in subsequent processes. The OFF characteristics of the transistor for a pixel become more favorable by reducing the film thickness of at least a region to be used as an active region of the display unit in the crystalline silicon film.
In one embodiment of the present invention, in the step of forming an amorphous silicon film on the whole surface of the substrate, the thickness of the amorphous silicon film is in the range of 100-150 nm.
According to the method of manufacturing a semiconductor device of the above embodiment, sufficient crystal growth can be obtained by making the thickness of the amorphous silicon film 100 nm or larger, and when the initial thickness of the amorphous silicon film before the film thickness is reduced is larger, both the ON and OFF characteristics of the transistor become more favorable. On the other hand, when the thickness of the amorphous silicon film exceeding 150 nm is not favorable in view of manufacturing costs since time is required to form an amorphous silicon film and reduce the film thickness later.
In one embodiment of the present invention, in the step of reducing the film thickness of at least a region to be used as the active region of a display unit in the crystalline silicon film, the film thickness is reduced by thermal oxidation.
According to the method of manufacturing a semiconductor device of the above embodiment, reduction of the thickness of the crystalline silicon film by thermal oxidation is more effective than silicon film reduction by dry etching since defects, impurities or the like in silicon Si can be reduced.
In one embodiment of the present invention, in the step of reducing the film thickness of at least a region to be used as the active region of a display unit in the crystalline silicon film, the thickness of the reduced crystalline silicon film is in the range of 20-60 nm.
According to the method of manufacturing a semiconductor device of the above embodiment, favorable OFF-current characteristics can be obtained by making the thickness of the crystalline silicon film in the range of 20-60 nm.